Glaucoma is a disease of the eye which is generally characterized by increased intraocular pressure within the eyeball and a progressive loss of vision. Typically, the treatment of glaucoma relies on the creation of a drainage channel which allows for the outflow of aqueous fluid from the eye. Heretofore, this has been mostly accomplished mechanically using instruments which cut externally into the sclera and its overlying episclera and conjunctiva. As an alternative to mechanical instruments which cut into the sclera, experimental laser treatments indicate that laser systems show promise.
Laser methods for use in the treatment of medical diseases are limited by the ability of the laser light to reach the tissue that is to be treated. Stated differently, depending on where the tissue is located, the tissue that is to be treated needs to either be exposed, or be at least somewhat transparent to the surgical laser beam that is to be used, or lie under a transparent layer of tissue. It happens that the transparency of tissue is dependent on its physical properties which affect the absorption and scattering of light by the tissue. Some of the specific factors affecting the transmission of light through biological tissue are age, medical condition, surface quality, hydration, temperature, and fluid content. In the context of the present invention, absorption relates to the tissues capacity to remove energy from the radiation and convert it into another form of energy. On the other hand, scattering is a general term for the irregular reflection or dispersal of light waves. With this in mind, it will be appreciated that absorption and scattering interact, with different results. In the case of a focused beam, absorption alone determines the maximum amount of energy that may reach a focal point. On the other hand, scattering determines the smallest possible focusable spot size.
As is well known, and obvious, the cornea of the eye is transparent to visible light. The sclera and the limbus, however, are not normally transparent to visible light. The limbus is the transition between the cornea and sclera and from the point of view of this transition it can be regarded as sclera. Nevertheless, it is interesting to note that the tissues of both the sclera and the cornea have similar anatomical structures. The main difference between the two is that the collagen fibers of the cornea have different widths and different separation or spacing than do the collagen fibers of the sclera. The consequence of this is that, unlike the cornea, the sclera effectively scatters light.
Heretofore, laser surgery on the sclera has been primarily accomplished using ab externo procedures which ablate tissue from the external surface of the sclera. These ab externo procedures have been done either by directing the laser light through the conjunctiva, or directly via a probe that is introduced under the conjunctiva. Ab externo procedures, however, can adversely disrupt the overlying conjunctiva and episclera. The only alternative, till now, has been ab intemo procedures which are employed to ablate tissue from the internal surface of the sclera. Presently used Ab inferno procedures are accomplished either by directing the light through the cornea with a contact lens or directly via a probe that is introduced into the anterior chamber of the eye. Such procedures, however, are labor intensive and not reliably efficacious.
In a radically different approach to this problem, the present invention recognizes that new ab interno procedures are possible by directly focusing light through the sclera or the limbus of the cornea. To do this, the new ab inferno procedures of the present invention effectively avoid the severe scattering of light that is caused by the sclera under normal conditions. The first method is to select and use wavelengths for the laser beams which are effectively transparent to the sclera. The second is to decrease the tissue index mismatch and thereby widen the bandwidth of sclera transparency to include light in the visible and near infrared range.
A third method may be used which effectively combines the above-mentioned first and second methods. For all methods of the present invention it is to be appreciated that ultrashort laser pulses are to be used (e.g. picosecond or femtosecond pulses) in order to make better incisions. This result is achieved because ultrashort laser pulses will have deterministic thresholds and will cause much less of the collateral damage which is believed to be responsible for undesired healing responses in surgical operations for glaucoma.
In light of the above, it is an object of the present invention to provide a method for ab inferno transscleral photodisruption of tissue which operates at wavelengths for which the sclera is effectively transparent. It is another object of the present invention to provide a method for ab inferno transscleral photodisruption of tissue which involves treating the sclera to make the sclera effectively transparent to the wavelengths of the laser beams. Still another object of the present invention is to provide a device for ab interno transscleral photodisruption of tissue. Yet another object of the present invention is to provide a method and device for the ab interno transscleral photodisruption of tissue which is relatively easy to manufacture and simple to use, as well as being comparatively cost effective.